A fuel cell mounted on a fuel cell car or the like generates a power by a chemical reaction between hydrogen in a fuel gas fed to an anode and oxygen in an oxidizing gas fed to a cathode. In general, a fuel off gas discharged from the fuel cell passes through a hydrogen dilution unit, and is discharged to the atmosphere in a state in which a hydrogen concentration is lowered. On the other hand, an oxidizing off gas discharged from the fuel cell is discharged as it is to the atmosphere.
However, in a case where the fuel cell is operated to quickly raise the temperature of the fuel cell while a power generation efficiency is low, hydrogen (mainly pumping hydrogen) is sometimes generated in a cathode. In consequence, hydrogen might be included in the oxidizing off gas. However, the discharging of the oxidizing off gas including hydrogen as it is to the atmosphere is environmentally unfavorable.
In view of such a situation, in a fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2006-73501 (paragraphs [0025] and [0093]), a feed channel of the oxidizing gas is connected to a discharge channel of the oxidizing off gas via a bypass channel. The oxidizing gas is introduced into the oxidizing off gas through the bypass channel such that the concentration of hydrogen in the oxidizing off gas is lowered.